At present, portable small electronic devices such as personal computers, cellular phones, and video recorders equipped with camera are widely used, and a small sized secondary battery having light weight and high electric capacity is desired to provide an electric source for driving such small electronic devices. From the viewpoints of small size, light weight, and high electric capacity, a lithium secondary battery is paid attention.
The lithium secondary battery employs a positive active electrode material comprising a complex oxide such as lithium cobaltate, lithium nickelate, or lithium manganate, a negative active electrode material comprising a carbonaceous material such as graphite into which lithium ions are able to intercalate and from which lithium ions are able to escape, and a non-aqueous electrolytic solution of a lithium salt in a non-aqueous solvent comprising a cyclic carbonate and a linear chain carbonate. The lithium secondary battery is now studied for improving its performances.
The lithium secondary battery employing LiCoO2, LiMn2O4, or LiNiO2 as the positive electrode active material is generally used under such condition that the electric charge-discharge procedure is repeated in the range up to the maximum operating voltage exceeding 4.1 V. In the procedure, the conventional lithium secondary battery is apt to gradually lower in its electric capacity when the charge-discharge cycle is repeated for a long period. It is supposed that this trouble is caused by oxidative decomposition of a portion of the non-aqueous solvent of the electrolytic solution on the surface of the positive electrode when the maximum operating voltage exceeds 4.1 V, and the decomposition product disturbs the desired electrochemical reaction in the battery. Therefore, the conventional lithium secondary batteries are not satisfactory in their battery performances such as the cycling performance and an electric capacity when the batteries are operated in the charge-discharge cycles of which maximum operating voltage exceeds 4.1 V.
U.S. Pat. No. 5,879,834 describes incorporation of an aromatic additive such as biphenyl, 1,3-chlorothiophene, or furan into a non-aqueous rechargeable lithium battery. The additive is used in an amount of about 1% to 4%. The aromatic additive is electrochemically polymerized at abnormally high voltages, thereby increasing the internal resistance of the battery and thus protecting it.
U.S. Pat. No. 6,074,777 describes incorporation of an aromatic additive such as phenyl-R-phenyl compounds (R=aliphatic hydrocarbon), fluorine-substituted biphenyl compounds, or 3-thiopheneacetonitrile into a non-aqueous rechargeable lithium battery. The additive is preferably used in an amount of about 2.5%.
It is an object of the invention to provide a non-aqueous electrolytic solution which is favorably employable for a lithium secondary battery and which shows high battery performances such as high electric capacity and high cycling performance, particularly, under the conditions that the maximum operating voltage exceeds 4.1 V and/or the battery is used at high temperatures such as 40° C. or higher.
It is another object of the invention to provide a lithium secondary battery which shows high battery performances such as high electric capacity and high cycling performance, particularly, under the conditions that the maximum operating voltage exceeds 4.1 V and/or the battery is used at high temperatures such as 40° C. or higher.